The conventional optical disks are practical and popular in optical recording media with fine storage quality and high stability, which have been widely utilized for data storage and multimedia entertainment. Accompanying with the advanced technological development, a mass amount of disks are produced into lots of categories and features, mainly divided into three types, read-only, write-once, and rewritable. The read-only type disks are CD-DA, CD-ROM, CD-I, VCD, DVD-ROM, DVD-Video etc. The write-once type disks are CD-R, DVD-R and so on. The rewritable disks are MD, MO, PD, CD-RW, DVD-RW, CD-RAM etc.
A pre-recorded disk is also called a read-only disk. The fabrication of read-only disk is to lithograph pre-recorded recording-pits as the recorded marks on a transparent substrate and then subsequently coated the reflection layer and protection layer by sputtering or spin-coating. The recorded marks can be read and retrieved by the reflected optical signals of the focused laser light delivered and collected by the pick-up head lens of the optical disk drive. Currently, the distance between the optical disk and the pick-up head lens is much larger than the wavelength used by the optical disks and disk drives commercially available. That means the optical recording technology is using far-field optics alone, it is unavoidable that an optical interference or diffraction phenomena will occur due to the wave characteristics of optics, and the spatial resolution of recording and reading is limited by the optical diffraction limit (i.e. 1.22λ/2n sin θ, where λ is the wavelength of the light used, n is the refractive index of the medium, and θ is the half angle of the aperture). In the past, the following methods were used to increase the recording capacity of the conventional optical disks;    (1) A more efficient coding and decoding technique.    (2) A smaller size of all the pits and their pitches of the tracks on optical disks.    (3) Use of the shorter wavelength of light source.    (4) Increase of the numerical aperture value of the objective lens.    (5) Use of the volumetric technology such as multi-layer recording, holography, etc.
Aforementioned methods are only the optimizations under the diffraction limit of far-field optics. A most basic way to improve the recording density and break through the diffraction limit is the use of the near-field optical technology. Eric Betzig of the Bell Laboratory, USA, first demonstrated the near-field optical recording using an optical fiber probe in 1992. His results overcome the optical diffraction limit. The recorded density was effectively improved. An optical fiber probe with an aperture of several tens of nanometer at the fiber end is used for the near-field optical recording and readout on a multi-layered platinum (Pt) and cobalt (Co) magneto-optical medium layer in his work. By controlling the fiber probe in a very close distance which is much smaller than the wavelength used for the experiments, an ultrahigh density recording of 45 Giga-bits per square inch was achieved. However, there are several difficulties and disadvantages of using the near-field fiber probe such as the precise control of the distance between the fiber probe and surface of the recording medium (about a few nanometers), the fragility of the fiber probe, low scanning speed, low optical throughput and high optical attenuation (around 10−6 to 10−3), and complexity of the fabrication of the nanometer-scale aperture at the end of the fiber probe.
On the other hand, an issued U.S. Pat. No. 5,125,750, disclosed a solid immersion lens (SIL) prototype that was possible and practical to implement the near-field disk drivers by G. S. Kino and his research team on the Stanford University, USA. The method of said patent has a reading/writing head which made of the semi-spherical and the super semi-spherical transparent solids—which have a high refection index, n,—for effective shrinking the reading/writing marks. Thus, said method of optical head could be speeding a reading/writing rate, then by adopting the present disk technology to directly develop into the high density optical recording of near-field disk drivers. In 1995, a company named TeraStor in San Jose, Calif., USA adopted this patented technological SIL as a “flying” reading/writing pick-up head to the near-field optical recording disk drivers, and tried to produce a first optical disk drive in high density optical recording. This high-speed “flying” reading/writing pick-up head had to be effectively controlled under a near-field range. The technical problems of the reliability of the flying pick-up head in the optical near field finally hindered the further developments of the high density near-field optical disk driver.
The issued patents of U.S. Pat. Nos. 6,226,258; 6,242,157; 6,319,582 and 6,340,813, in which Dr. Junji Tominaga disclosed a design, by adding two nano-film layers (SiN in 20 nm and Sb in 15 nm) onto the normally used phase-change optical disk to replace the near-field effect of an optical fiber probe of the near-field scanning microscope, and to carry out the read/write actions beyond the optical diffraction limit.
Aforesaid design show a usage of alternating of thin-film structure on the disks to reach a near-field ultra high density of optical recording. Then accompanying with an improved structure of the film layer of said disks, said structure improved the two main structures of said film layer from a first category (Sb and SiNx1) to a second category (AgOx and ZnS—SiO2). However, said film layer of said two categories, which generated a localized near-field optical effect of Sb and AgOx nano-film layer, of their substances/materials are unstable, and can easily lose the properties of localization due to high temperature and the absorption of water vapor.
The present invention is a read-only near-field optical disk with a zinc-oxide (ZnO) nano-structured thin film and a spacer layer such as ZnS—SiO2 on the pre-recorded substrate. The ultrahigh density read-only near-field recording disk can be effectively achieved by this invention.
In summary, aforementioned conventional far-field optical method appears that the short-wavelength of light-source is costly, and the reading/writing spots of a conventional disk driver have an optical diffraction limit, so only the near-field optics with no diffraction limits can effectively improve the recording spot size below the diffraction limits. Additionally, the near-field optical technique of aforesaid near-field scanning probe and SIL near-field optical disk drive have lots of difficulties, which makes said near-field optical disk become an appropriate choice for near-field optical recording. It is known that Sb and AgOx are unstable substances/materials for manufacturing disks, so this invention uses more stable and better localized near-field optical effect of zinc-oxide (ZnO) nano-structured thin film(s) to produce the read-only zinc-oxide (ZnO) near-field optical disks. This invention is to use the stability and the localization effect of the zinc-oxide (ZnO) nano-structured thin film along with a near-field spacer layer of ZnS—SiO2 to achieve an ultrahigh density near-field optical disk. The localized near-field optical effects can be happened between the zinc-oxide (ZnO) nano-structured thin film and pre-recorded pits or marks on a transparent substrate in near-field range, There is no diffraction limit for the read-only optical storage using this method.